研究興趣: 1. 半導體和奈米材料之製備、分析及化學反應

Synthesis, Characterization and Chemical Reactions of Nanomaterials

2. 半導體太陽能材料之製備和性質鑑測

Preparation and Characterization of Semiconductor Solar Cells

3. 材料表面性質、催化反應和電化學

Properties and Catalytic and Electrochemical Reactions of Materials Surfaces

4. 超高真空表面分析技術之發展和應用

Development and Applications of UHV Surface Analysis Methods

5. 超級電容奈米材料之開發與分析

Development and Analysis of Nanostructures as Supercapacitor Materials

6. 生醫材料之製作、表面修飾及抗菌功能之探討

Study on Preparation, Characterization, and Anti-bacterial Function of Biomaterials

研究實例

I. Reduced dimensional nanomaterials 低維度奈米材料之合成和性質鑑測

Oxide-based nanomaterials have widespread applications in semiconductor electronics, magnetic sensing and memory devices, optical recording and telecommunications, energy conversion and storing, medical diagnosis and sensors, environmental monitoring of toxins and remediation, detection of biological and chemical agents, as well as in areas such as aerospace and aeronautic industries, public security, and personal safety. Depending on their chemical composition, dimensionality, and size, the oxide-based nanomaterials may have a bonding nature

that exhibits metallic, covalent, or ionic bonding character, and thus behave as conducting, semiconducting, or insulating materials.

During the last decade, the controlled synthesis and characterization of these nanomaterials, especially with reduced dimensionality, has been a fascinating objective in nanotechnology, materials science, chemistry, and physics. Our major goal of reduced-dimensional nanomaterials is the design of material structures/devices that could be easily integrated with modern nanoelectronic fabrication techniques. Through an inclusive multi-disciplinary approach, our design ensures tailoring of novel sensor, catalyst, and/or electronic switch properties and response of the structures in the nanoworld. A. Polycrystalline Nanoribbons and Their Growth Chemistry

For example, we developed a simple, solution-phase synthetic method to prepare a copper-based, one-dimensional nanostructure, in which cuprous compounds were used as the metal precursor. The Cu2O nanoribbons produced after heating the solution at a specific temperature were dispersed to some extent in the reaction medium. The reaction temperature was low, as compared to some reports in the literature on producing Cu2O nanostructures of various morphologies. In addition, the method has the advantages of low cost and possible high-quantity production. It is an efficient method since the solvent used in the synthesis also acts both as a mediator and as a reducing agent. X-ray photoelectron spectroscopy (XPS) confirmed that the chemical composition of the nanostructures we synthesized using the method was mainly Cu+ or Cu0. The step-by-step, dimensional growth chemistry involved in the synthesis of the copper-based nanostructures was derived at the molecular level for the first time using the growth of the Cu2O nanoribbon as an example.

Since many Cu2O applications such as lithium ion batteries prefer material structures of a large surface-to-volume ratio, the synthetic method we have developed that allows control of the axial growth of products in the reaction medium is expected to be versatile. Furthermore, Cu2O is a p-type semiconductor with a narrow indirect band gap of 2.2eV. The Cu2O nanoribbons we synthesized are expected to find applications in catalysis, gas sensors, energy conversion, and magnetic storage because of the nanostructures’ unique chemical, optical, and magnetic properties. It should be pointed out that Cu2O structures have been used in CO oxidation and nitrite sensing. They also act as gas-phase sensors for gasoline and alcohol, as catalysts for click reaction between acetylenes and azides, and as high-capacity and high-cycle retention anodes in lithium ion batteries.

Figure IA-1. (a) Low-magnification SEM, (b) high-magnification SEM, and (c) TEM images and (d) EELS mapping (green- carbon; red- copper element) of the nanoribbons obtained from the reaction of CuCl in EG in the presence of PVP.

Figure IA-2. (a) TEM image and (d) AFM image and the topographic line scan (inset) of the nanoribbons obtained from the reaction of CuCl in EG in the presence of PVP; (b) HRTEM image taken of the marked position shown in (a); and (c) simulated SAED pattern of Cu2O overlaid on the experimental pattern of the ribbons synthesized.

Scheme IA-1. Schematic illustrations of the growth of the ribbon nanostructures from EG and CuCl at elevated temperature in an oxygen environment. The chemistry involved in the growth includes disproportionation of Cu+ and nucleophilic substitutions of EG with chelated glycolates. B. Rational Shape-Controlled Synthesis of Mesostructures

Fabrication of micro- and nano-structures with specific geometries offers the possibility of manipulating electromagnetic fields on the meso-scopic level. In optoelectronics, structures in the shape of nanopillars, when patterned, can increase light absorption for photovoltaic performance. Concentric meso-rings can enhance local fields and are potential candidates for linearly or radially polarized excitation. Crossed networks of ZnO nanorods and chromophore-embedded nanoplates are promising for optical filters. A parallel-plate structure applied with a time-varying voltage and a large diameter cylindrical quantum dot that is comprised of two wells separated by a thin barrier may be used as electro-optic absorption modulators. Contrived mesostructural devices of proper geometries can thus effectively manipulate the property and the propagation of light.

Among the structures with geometries promising for use in integrated optoelectronics, rhombs have rarely been synthesized, even though most achromatic quarter-wave plates are rhomb-type devices. A wide range of applications in compact low-pass filter, remote sensing, and astronomy, for example, can be envisaged if two rhombs of quarter-wave phase shifts are combined to give an achromatic half-wave retarder. The phase retardation varies with wavelength and with acceptance

angle for different rhomb designs, such as length-to-aperture ratio, surface coatings, and strain birefringence.

We successfully develop a facile chemical method that used two polyols to generate nearly monodispersed metal-organic rhombic platelets. The success lies on our careful selection of precursors of proper oxidation states and our meticulous control and the optimization of both the thermodynamic and the kinetic conditions for synthesis. Cuprous acetate, which acted as a heterogeneous nucleation agent, was dispersed in ethylene glycol, which acted as a stabilizer, a ligand, and a monomer for the formation of polymeric glycolates. By adjusting the volume ratio of polyethylene glycol (PEG) to ethylene glycol and the polymer size of PEG, rhombic platelets of 80 – 180 nm in thickness were synthesized with aid of suitable structure-directing and dispersing agents. Energy-dispersive x-ray spectroscopy and FT-IR analyses revealed that the rhombic platelets were mainly composed of copper glycolate polymer chains. Knowledge obtained from this study can be expected to be applied to and to shed light on broad research topics concerning novel metal-organic nanostructure syntheses.

Figure IB-1. Self-assembled copper glycolate meso-rhombic platelets of high monodispersity were synthesized in ethylene glycol solutions. They had 170–440 nm in side length and 80–180 nm in thickness.

Figure IB-2. EDX spectrum of the precipitates produced from the reaction of cuprous acetate in a PEG400/ethylene glycol (1/7, vol/vol) solution at 150ºC for 2 hrs in the presence of PVP.

Figure IB-3. Representative (a) x20,000 and (b) x40,000 magnification SEM of the precipitates obtained from the reaction of cuprous acetate in a PEG400/ethylene glycol (1/7, vol/vol) solution at 150ºC for 2 hrs in the presence of PVP.

Figure IB-4. SEM micrographs of the precipitates obtained from the reactions of cuprous acetate in a) PEG200/ethylene glycol (1/7, vol/vol), b) PEG1500/ethylene glycol (1/7, vol/vol), and c) PEG3000/ethylene glycol (1/7, vol/vol), respectively, at 150ºC for 2 hrs in the presence of PVP.

II. Integrated-circuit interconnect chemistry 積體電路導線化學 A. Dewetting of Copper Nanolayers on Silica: Toward Preparation of Copper Meso/nanowires by Self-organization

Copper is an excellent material for interconnects in integrated circuits (ICs) due to its high bulk thermal and electrical conductivities and low electromigration rates. However, copper is known to bind poorly to silica. The dewetting of copper interconnects due to heat generated from repeated operations of the IC may affect the properties, performance and reliance of the copper wires as well

as the circuit. To simulate the operation conditions in the real world, heating experiments of copper on silica have to be performed in oxygen so as to clarify the role oxygen plays in the copper dewetting process.

We studied the dewetting behavior of copper nanolayers on silica during annealing in an oxygen environment and explored the interaction of copper with silica and silicon. The dewetting of copper obtained in oxygen was quite different from that observed in the vacuum. The dewetting also depended strongly on the oxygen pressure used. More importantly, copper could self-organize to form line structures after the copper-deposited silica sample was annealed in a specific range of oxygen pressure. In general, in-plane metal and metal-containing meso/nanowires are fabricated on solid substrates on which some forms of line features are present to guide the growth of the wires and to “pin” them to the substrate. The line structures mentioned above were prepared via a self-organized approach, since no specific experimental steps were taken to create surface line defects. The line orientations were determined by the surface morphology of the silicon substrate beneath the silica layer. Further studies are underway to fabricate copper interconnects via self-organization.

Figure IIA-1. SEM images taken from the Cu/silica sample prepared on a Si(100) substrate after the sample was annealed in oxygen of pressures (a) & (e) 10-2 torr, (b) & (f) 10-3 torr, (c) & (g) 10-4 torr, and (d) & (h) 10-5 torr, respectively. The magnification factor for the images shown in the upper row was 2,400X, and that for the lower row 20,000X.

Figure IIA -2. (a) AFM micrograph taken from a Cu/silica sample after it was annealed in oxygen; (b) the line profile for the white line across a line structure shown in (a).

Figure IIA -3. Cu 2p XPS spectra taken from the Cu/silica sample prepared on a Si(100) substrate after they were annealed in oxygen of different partial pressures. The composition of the resulting line structure was determined to be mainly metallic copper and Cu+.

Figure IIA -4. SEM images taken from the Cu/silica samples prepared on (a) Si(110) and (b) Si(111) wafer substrates, respectively, after they were annealed in oxygen. The copper line directions corresponding to the crystallographic orientations in the atomic structures of the Si(110) and Si(111) surfaces, respectively shown in (c) and (d), are indicated.

Figure IIA -5. SEM image taken, at a detection angle of 45º, from the exposed face of the Cu/silica sample after the sample was annealed in oxygen and sliced off in a manner perpendicular to the

orientation of the line structure formed on the surface. B. Metallization on Semiconductors in the Sub-10-nm Regime

Interest in devices having feature sizes in the single-digit nanometer regime is growing rapidly. In this regime, three independent physical length scales—those that govern quantum mechanics, electrostatics, and magnetism, respectively—conspire to determine the operation of electronic devices, such as quantum well lasers, single electron transistors, and magnetic hard disks. Future innovation and the high-volume manufacturing of devices having single-digit-nanometer-sized features will, therefore, open up a whole new world of novel applications far beyond those that current chips offer.

Achieving success in producing nanostructures in deep nanometer dimensions without addressing adequate metal contacts is, however, problematic. Given the prospect of extreme device densities in chips possessing single-digit-nanometer feature sizes, electronic devices fabricated based on sub-10-nm structures cannot function well with conducting channels prepared using top-down lithographic methods, such as damascene interconnect technology. Alternative signal transport approaches must be developed to interconnect, vertically or horizontally, these nanostructures within or between layers to allow communication.

We have made effort on exploring alternative metallization methods for interconnecting structures in ICs having sub-10-nm dimensions. Instead of using the top-down approach of electrodeposition and etching, atomic-scale metal contacts were formed in a bottom-up manner by chemically depositing organometallic compounds at low temperature. Our study distinguishes itself from those performed previously by using inorganic complexes of linear atomic metal strings in the deposition. With such a bottom-up approach, the property of the metal contact produced will be determined mainly by the nature of the chemical bond formed from individual metal stings at the contact and not by the production process, the degree of contamination, or complications due to the formation of a ternary system. Therefore, site-specific bonding of the metal-string complex allows not only strategic design of the nature of the contact but also detailed investigations into the bonding chemistry involved in the formation of the contact.

Figure IIB -1. Schematic depiction of the bonding of trinuclear chromium complexes on a semiconductor surface. Four 2,2´-dipyridylamino ligands were wrapped helically around the central linear Cr string.

Figure IIB -2. Positive (left) and negative (right) secondary ion mass spectra (SIMS) of a GaN(0001) surface exposed to the indicated doses of trichromium linear atomic metal strings.

Figure IIB -3. Positive secondary ion mass spectra (SIMS) of a GaN(0001) surface exposed to (a) 2,2´-dipyridylamino ligands, (b) C5D5N, and (c) trichromium linear atomic metal strings. The spectra have been normalized to a constant peak height at m/ e 69. Understanding the fragmentation patterns of small model molecules helps to characterize the chemical structure of the linear atomic metal string complex bonded on the substrate surface.

Figure IIB -4. (a) XPS spectra of Cr 3p obtained from the GaN(0001) surface exposed to the indicated doses of the linear atomic metal string complex and (b) the corresponding bonding orientation of the string complex on the substrate surface. The variation of the bonding structure of the linear metal string complex with the dosage may be derived by examining core-level photoelectron spectra of different elements. III. Electrochemistry with ultramicroelectrodes in the nano range 奈米尺度超微電極之電化學

The development of ultramicroelectrodes opens up a fertile range of research opportunities about exploring in detail electrochemical reaction mechanisms with a large kinetic range and about understanding the electrochemical steady-state phenomena. Ultramicroelectrodes are those having critical dimensions smaller than the diffusion layer formed in an electrochemical process. As the scan rate of a working electrode is increased, the diffusion-controlled peak current in response to a reversible redox couple diminishes compared to the charging current. The upper scan rate is also limited by distortions created from the increased current and the associated resistance. However, the ultramicroelectrode (UME) has a relatively large diffusion layer and a small overall current because of its small size. They allow the use of fast scan rates with limited distortion of the current

response, which in turn enables the study of electrochemical reaction mechanisms with a large kinetic range. In addition, linear sweep voltammograms in the steady-state region exhibit reversible redox couples as steps rather than peaks. When the scan rate of the regular electrode is dropped to enter the steady-state regime, the current measurement usually becomes unreliable. The high ratio of diffusion layer volume to electrode surface area of UMEs also facilitates their scan rates to be dropped to the steady-state regime with reliable current measurements.

We have recently developed a method to fabricate UME arrays. As shown in Figure IIIA-1, by virtue of this novel fabrication approach, cyclic voltammograms indicative of the presence of UMEA were obtained. The UME we fabricated on the silicon wafer had the critical dimension in the nanometer range. The small size of the UME in the UME arrays we fabricated gives the UME an unprecedented, relatively large diffusion layer and a very small overall current. These characters greatly increases the useful scan rate of the UME arrays we have designed to a very high value and thus allows the study of the electrochemical reaction mechanism with an enormous kinetic range. These UME arrays also display a great capability for ultratrace analysis.

Figure IIIA -1. SEM image of a portion of the fabricated UME array.

Figure IIIA -2. Cyclic voltammograms obtained in MeCN of 1mM ferrocene/100mM TBAP on (a) the original and the Pt-UMEA and (b) the electrodes prepared with deposition times of 500sec (UMEA) and 1 hr (regular electrode-like), respectively.

Figure IIIA -3. Differential pulse anodic stripping voltammograms of (a) Cd2+ and (b) a mixture of Cd2+ and Pb2+ obtained using the Bi-deposited UMEA that we designed as the working electrode. The enhancement of the signal intensity with the scan cycle revealed the potential of the designed ultramicroelectrodes for ultratrace analysis.

IV. Biomaterials performance 生醫材料研究 We also carry out collaborate research with colleagues in School of Dentistry, National Taiwan

University to investigate and enhance the performance of dental material. For example, a study has been carry out to reveal the effects of calcium hydroxide (Ca(OH)2) dressing in root canals and the effects of subsequent acid etching on the adhesion of luting resins to root canals. Root canal treated teeth with insufficient coronal structure generally require radicular posts for crown restoration. The traditional choice for anterior restoration involves cast metal posts and cores covered by porcelain fused to metal crowns. Fiber posts have recently been used to replace cast metal posts because their esthetic outcomes for anterior teeth are more pleasing. In addition, the elastic moduli of fiber posts are similar to those of dentin and provide an even distribution of stress along the teeth. Using adhesive luting resins to lute fiber posts can improve retention, reduce microleakage, and increase resistance to tooth fracture. Conventional luting resins require pretreating the canal surface by, for example, applying an etchant, primer, or adhesive before bonding the fiber post to the root canal wall. However, this multi-step application technique might compromise bonding effectiveness. Self-adhesive resin-based luting agents, such as Maxcem Elite (Kerr, Orange, CA, USA) and RelyX Unicem (3M ESPE, St Paul, MN, USA), combining the etchant, primer, and adhesive have thus been proposed. Without the time-consuming procedure of surface pretreatment, using self-adhesive luting resin has greatly shortened the clinical chairtime.

Ca(OH)2 is a widely used medicament for canal disinfection, but it is difficult to remove from the root canal completely before obturation. The effects of Ca(OH)2 dressing and subsequent acid etching on the bond strength of luting resin to root canal dentin have not yet been evaluated. In this exemplified study, root specimens were prepared from extracted human permanent molars. Specimen canals were 1) filled with etch-and-rinse (Nexus® third generation, NX3) and 2 self-adhesive (RelyX Unicem, Maxcem Elite) luting resins, respectively; 2) dressed with Ca(OH)2 before Ca(OH)2 removal and luting resin filling; 3) dressed with Ca(OH)2 before Ca(OH)2 removal and post cementation; or 4) treated as described in 2) except that the canals were further etched with phosphoric acid before luting resin filling. Push-out bond strengths were measured and analyzed using one-way analysis of variance (ANOVA), and Fisher’s multiple comparison tests provided a follow-up comparison among these four canal treatments. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) were used to analyze the specimen surfaces. Our analysis showed that Ca(OH)2 dressing adversely affected the bond strengths to canal dentin of the three luting resins tested. Acid etching did not increase the bond strengths. Infrared analysis revealed that Ca(OH)2 dressing caused no structural changes on the dentin surface. XPS and SEM analyses revealed Ca(OH)2 remnants as the ultimate chemical cause leading to the decrease in bond strength. The bond strength of luting resin to dentin was thus affected by Ca(OH)2 dressing. Acid etching treatment could not increase the bond strength. Adhesion of the fiber post to the root canal wall may be compromised after Ca(OH)2 dressing. An effective method for complete

removal of Ca(OH)2 dressing or increase of bond strength for luting resin needs to be developed.

Figure IV-1. FTIR spectra of dentin taken before (a) and after (b) 1, (c) 3, and (d) 5 weeks of Ca(OH)2 treatment.

Figure IV-2. Ca 2p and O 1s XPS spectra of (a) Ca(OH)2, (b) root canal dentin without Ca(OH)2 treatment, (c) dentin receiving the Ca(OH)2 treatment, and (d) dentin receiving acid etching after the Ca(OH)2 treatment.